During the first year of grant support it was demonstrated that the requirement for extracellular Ca2 ion for the effects of ADH on osmotic permeability of toad bladder actually reflects a tissue requirement and that ADH can successfully increase water permeability. It has been shown by others that extracellular Ca2 ion is not required for the effects of ADH on Na ion transport. These studies are going to be extended also to the effects of ADH on urea permeability. A study in depth will be conducted to reevaluate the roles of Na ion and K ion on the effects of ADH on water permeability. The effects of serosal hypertonicity on water permeability and its interaction with ADH will be evaluated.